1. Field of the Invention
The invention broadly relates to optical servo systems. More particularly, the invention relates to the optical detection system used in an optically assisted disk drive to detect marks on a magnetic disk and thereby precisely locate the magnetic read/write head relative to tracks on the magneto-optical disk.
2. Brief Description of the Prior Art
Since the introduction of the personal computer in the 1970""s and the development of the floppy disk, the need for greater and greater amounts of storage space has continued unabated. The original floppy disk could store less than 100 kilobytes and the most commonly used (3.5 inch) floppy disk today, introduced in the late 1980""s can store 1.4 megabytes. Although fixed (hard) disks now store many gigabytes, there remains a need for removable storage media with high capacity.
High capacity removable storage media became popular in the 1980""s with the advent of desktop publishing (DTP). Relatively large, clumsy, and undependable xe2x80x9ccartridgesxe2x80x9d from Syguest, Iomega, and other companies were used to transport large DTP files that could not fit on a floppy disk, to a printing plant. High capacity storage media is still in demand today for transporting large files when a broadband connection is not available and for transporting confidential information without using the public network.
One high capacity removable media system which is growing in popularity is the xe2x80x9ca:drivexe2x80x9d from OR Technology Inc. of Campbell, Calif. While its outward appearance is almost indistinguishable from that of a 3.5 inch, 1.44 megabyte floppy disk drive, the xe2x80x9ca:drivexe2x80x9d provides 120 megabytes of storage on ultra high density disks, known as xe2x80x9cLS-120xe2x80x9d or xe2x80x9cSuperdiskxe2x80x9d media. At the same time, the xe2x80x9ca:drivexe2x80x9d product is compatible with current and legacy 3.5 inch technology and can read and write to both 720 kilobyte and 1.44 megabyte disks. As its name implies, the xe2x80x9ca:drivexe2x80x9d can serve as a bootable drive in any system in which it is installed.
The xe2x80x9ca:drivexe2x80x9d achieves its high capacity and enhanced accuracy and reliability by using an optical positioning system for accurately guiding a magnetic dual-gap head that accommodates the differing track densities of conventional and ultra high density disks without error or mishap.
Prior art FIG. 1 shows a dual media disk storage system for reading data from and writing data to the surface 10 of removable magnetic media 12 having an axis of rotation 14 and a plurality of concentric data tracks 16. Although the disk drive system is capable of handling dual media, in this instance, for the sake of clarity, only one disk is shown, the well known 3.5 high density type that holds 1.44 megabytes when formatted.
A read/write head 18 is guided by an actuator 20 and actuator arm 22 which positions the read/write head 18 over a desired track 16 on the surface 10 of disk 12. The actuator arm 22 carries a strip having a periodic reflection profile 24 which is used in this instance because the 3.5 disks do not carry any location markings on their surface. In this instance, the periodic reflection profile 24 is a linear encoder. Actuator 20 is under control of a conventional, closed loop servo system 28 which is responsive to a signal from an optical sensor 30 mounted on the underside of sensor housing 28.
FIG. 2 shows in more detail how a split beam arrangement is used to detect either the reflection profile for a linear encoder when reading/writing 3.5 disks or the markings on the surface of an LS-120 disk when reading/writing it. The sensor system carried on the arm 22 includes, in addition to the light detector 30, a laser source 32, a hologram 34, a lens array 36 and a rooftop mirror 38. Light from the laser source 32 is diffracted by the hologram 34 and focused by the lens array 36. The rooftop mirror directs the light and reflections to either the linear encoder 24 or the surface of an LS-120 disk 40.
It can be appreciated from prior art FIG. 2 that the sensor system requires multiple passive optical elements, all of which must be aligned during the assembly process. The alignment requires expensive tooling. Each passive element occupies a finite space and additional space must be provided for the alignment tooling. The sizes of the elements also require a large mechanical supporting structure.
In addition, it will be appreciated by those skilled in servo system shown in prior art FIG. 2 is relatively large and wi a xe2x80x9cfull heightxe2x80x9d drive bay to be accommodated.
Additionally those skilled in the art will recognize th servo system for an LS-120 type disk drive requires a quadrature between the adjacent sensors; that the detection of the sensors m synchronous; and that the first stage of the pre-amplifier used in a typical optical servo system would be more effective by amplifying the tangential and radial tracking signals only and not the DC component, thereby allowing for a larger gain and more signal amplitude.
In the prior art, light from a laser is typically focused by passive optical elements to form three highly focused points of light at the plane of the disk. Other passive optical elements gather reflected light from these three points and focus the reflected light onto three separate detectors. The detectors produce track sensing signals that include a low noise pair of sinusoidal signals in exact quadrature indicating the track radial position.
It is therefore an object of the invention to provide an improved positioning device (referred to hereinafter generically as optical servo systems) that can be used for metrology, optical data storage, or data storage systems that use optical features on the media for tracking purposes.
It is also an object of the invention to provide optical servo systems for data storage devices that do not require alignment of elements during assembly of the data storage devices.
It is another object of the invention to provide an optical servo system for a data storage device that reduces the overall cost of the data storage device.
It is still another object of the invention to provide an optical servo system for a data storage device that is smaller in size than conventional optical pickup systems.
It is also an object of the invention to provide an optical servo system that provides built-in quadrature phase shift.
It is another object of the invention to provide an optical servo system in which the source and detector(s) are fabricated on a single substrate.
It is still another object of the invention to provide an optical servo system in which each of the optical detectors and apertures are small enough and close enough to provide a synchronous detection.
It is also an object of the invention to provide an optical servo system that requires fewer components in the first stage of a signal preamplifier.
It is another object of the invention to provide an optical servo system that operates without lenses.
It is still another object of the invention to provide an optical servo system that has a high signal to noise ratio.
In accord with these objects, which will be discussed in detail below, the present invention provides a lensless optical servo system having an unfocused light source and patterned photodetectors. The unfocused light is reflected by the markings on an LS-120 disk and the reflected light carries the pattern of the markings the considerable distance in its far-field to the photodetectors. The convolution of this light pattern and a mating geometric pattern on the photodetectors causes the photodetectors to generate signals representing the position of the track on the disk.
According to a presently preferred embodiment of the invention, set forth herein to illustrate the principals of the invention, a laser diode and three detectors are formed on the same silicon substrate. Further, according this illustrative preferred embodiment, sinusoidal metalization (a form of aperture) is applied to the detectors in the radial direction (radial relative to the LS-120 disk); the period of the sinusoidal metalization is approximately two times the tracking pitch of the disk; the metalization on the first detector is offset radially approximately ninety degrees behind the metalization on the second detector; and the metalization on the third detector is offset radially approximately ninety degrees ahead of the metalization on the second detector. Alternatively, in lieu of sinusoidal metalization an absorbing sinusoidal feature may be provided.
Preferably, each detector is provided with two sinusoidal patterns, approximately one hundred eighty degrees out of phase with each other with respect to the tangential direction (tangential relative to the LS-120 disk). The patterns are subtracted to remove the DC component (noise) of the tracking signal. This allows for a differential detection system to occur before the tangential modulation of the tracking signal is removed. Thus, the first stage of a pre-amplifier is more effective by amplifying the tangential and radial tracking signals only and not the DC component, thereby allowing for a larger gain and more signal amplitude.
The silicon substrate is also provided with a source laser diode that is aligned so the reflection from the disk is substantially centered on the second detector. A fold mirror is also provided to direct light from the source laser diode to the disc in a manner well known by those skilled in the art.
The invention features an positioning systems, (such as optical servo systems), which perform optical sensor functions without the need for an imaging optic, utilizing only unfocused laser light and patterned photodetectors.
Advantages of the invention include (1) obviation of separate holographic optical elements required by the prior art (as well as other lens elements) to reduce system cost; (2) a less complex assembly process for the devices contemplated by the invention is achieved by (a) eliminating the need to align and attach any optical elements to the laser detector module, (b) eliminating the need to steer light onto a detector during the assembly process since all reflected light from the media will have all the tracking information required to operate the system, and (c) since the radial shift required to make the tracking signals in quadrature is built into the aperture, there is no alignment requirement for this parameter; and (3) end users experience a lesser degree of complexity and cost in installing the device contemplated by the invention in their products since there is less material physically present at the top of the detector system which nominally requires delicate handling.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.